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What You Need to Know about Black Cutworms

18:57PM May 03, 2012


As corn begins emerging across the Corn Belt, unseasonably warm weather has farmers on high alert for black cutworms.
Iowa farmer and agronomist Jason Franck says that the mild winter and increased weed pressure in March could likely result in increased black cutworm pressure that farmers in his area haven’t seen for a few years. Kelly Estes, coordinator of the Illinois Cooperative Agriculture Pest Survey, agrees that black cutworms will be of concern for farmers in the Midwest this year. She says that significant moth flights in March and April could result in increased pressure from the bugs throughout Illinois in the coming weeks.
Moths are the cause of cutworm infestation. In the early spring, moths search for green fields to lay their eggs. Often those are fields that are weeded over and haven’t been worked yet. Once the eggs are laid by the moth, it takes 300 heat units for the eggs to hatch and develop into larvae, according to Farm Journal Field Agronomist Missy Bauer. It is when the larvae reach stage four of development that they begin to do the most damage on corn plants, Estes says. This is often at time of emergence.


Scout It Out

Bauer says the first step to scouting for the pest is to identify high-risk fields. "Think back, when moths were flying in late March and April, which fields were green?" she says. Next, figure out when heavy pressure will occur. "Look at your local university data to see when heavy flight occurred," Bauer says. Then, count 300 heat units from that date to figure out when to scout, she says. Head to high-risk fields to look at emerged to v5 corn. "The worm will cut the plant in two at the base of the plant, right at the soil," Bauer says. Leaf damage can also be caused by black cutworms. Estes encourages farmers in Illinois to begin scouting in the next few weeks.

A Low Threshold Pest

Bauer says that while black cutworm thresholds vary from state to state, they are low across the board. "In our state it is only 3% to 5%," she says. "So if you’ve got 3,000 plants in your stand count and you only find one plant damaged or cut, you’re at the threshold." Local university extension offices can provide farmers with current threshold data for the black cutworm in their area.
John Obermeyer, an integrated pest management specialist at Purdue University, says that last year many farmers in Indiana learned about black cutworms the hard way because of an infestation throughout the state. He says that many producers discovered that under high pressure many seed-applied insecticides did not offer control. They also discovered that "some varieties of Bt corn do not perform well; those are cases where the label provides only ‘suppression’ and not ‘control,’" Obermeyer says. He encourages producers to read the fine print and to treat their fields infested in the 3% to 5% threshold range with a foliar insecticide.